ClaudeAdapter token usage: residual fallback to accumulated totals when no stream events arrive

[harrryyd]

Problem

PR #126 fixes the main token usage over-reporting bug by capturing per-request usage from message_start/message_delta stream events. However, completeTurn still falls back to accumulated totals from result.usage when lastKnownTokenUsage is undefined — which happens when no stream events arrive (e.g., non-streaming mode, or API calls that dont stream).

// ClaudeAdapter.ts completeTurn (approx line 1506-1533)
const accumulatedSnapshot = normalizeClaudeTokenUsage(
  result?.usage,
  resultContextWindow ?? context.lastKnownContextWindow,
);
// Falls back to context.lastKnownTokenUsage, 
// which could be accumulatedSnapshot if no stream events

Acceptance criteria

• Determine when stream events might be absent (non-streaming mode? edge cases?)
• Either handle the fallback accurately or document that it is acceptable
• Consider whether result.usage can be used differently in non-streaming scenarios

Related

• Bug report: token-usage-bug-report.html (root of repo) — contains detailed analysis of all three bug paths
• Fix PR: fix: per-request stream usage for context window occupancy #126 (addresses the two streaming paths, not this fallback)
• Original report drafted during PR feat: provider compaction + context trim + archive-and-new + review flow #108 review